Attenuator



i938 P. J. HERBST ET AL 2, I

ATTENUATOR Filed Sept. 28, 19255 2 Sheets-Sheet l 75' 7 7 INVENTOR RIHERBT, RHHEACOCK, P.G.B|NDLO$S WQXM ATTORNEY Auge 1938. P. J. HERBST ET AL ATTENUATOR Filed Sept. 28, 1935 2 Sheets-Sheet 2 m RE oHm Mm m m m Q o V ,8 Nfl W BP A R Patented Aug. 2, 1938 ATIENUATOR Philip J. Herbst, Merchantvill and Philip G. assignors to Radio ma, a corporation of Dela Application September 28, 1935, Serial No. 42,592

cock, Oaklyn, bl. J.,

2 Claims.

This invention relates to attenuator networks such as are utilized in connection with the microphone mixer panels of sound recording apparatus and the like, and has for its principal object the 5 provision of an improved attenuator device and method of operation whereby the construction of such devices is simplified and the extraneous noises incident to their operation is reduced to a minimum.

been observed that a leakage current of a small amount due to the in potential then ground level.

e, Ralph H. Hea- Bindloss, Audubon, rporation oi Amerware put circuit being at higher greatly increased the noise By using two similar metals as contacts such 5 as nickel steel against nickel ste thermal currents should be genera tro-thermal difierence between t is zero. However, such combina el, no electroted as the elecwo such metals tions resulted in Attenuator devices usually depend for their 1'0 operation on the use of moving contacts which noise levels which were apparently dependent tend to g nerate objectionable noise. The exact upon the condition of the the nature of cause of such noise is diflicult to determine for the P Contact and the relative veloclty of the metals reason that its results from a number of causes m which tend to mask one another. In general, the In our Investigation of this phenomena three sources of such noise ber of different metals were used as contactsin an apparatus consisting of a motor driven insu- (1) IntermPtmn of exlstmg currents' lated disk to which a polished disk of one of the (2) Production of electro-thermal currents. metals to be investigated was bolted and a com 20 (3) Agitation of the electrons in the contacts. tact arm carrying two brushes of a Second metal The noise producing currents resulting from a, The contact arms were connected in series with change in circuit connections may be electro- 250 ohms to the input of a microphone amplifier. thermal, may result from leakage or may be due The output of this amplifier was fed into an amto electromagnetic or electrostatic pick-up. plifler, the output of which operated a volume 25 Careful shielding of the attenuator input circuits indicator meter. This apparatus was calibrated and the use of blocking condensers in the various after each set of readings. The results are tabu-, leads do not appreciably reduce the noise. It has lated below:

Thermo- S 11 t 8 in N l l l Rotor material ptgtegzlfiligl aga Brush material pejlgecgiigl Egg ff g Nickel steel -26 30t033 122 35 opper +2. 75 1.72 Brm 140 no? Phos. bronze l52 Cop. carbon. -140 sn 1.622 n -140 1.622 0) to as -12o -145 1.622 m 2.6 Above 10ll 9.78 Above -100 1.622 -12@ -12e 9.78 121 1 $5 335 a 18 -14s 50 9.7a -1s2 -1l8 Noise too low to be measured.

Noise too low to be measured.

It will be noted that whenever two metals having high thermoelectric potential difference are used, the resulting noise level is high. However, this does not account for the relatively high noise levels when like metals are used,on disk and brushes as in the case of nickel silver, chromium and iron. Even such metals of silver and copper make noisy contacts if the surfaces are not smooth. It will be noted that the noise level is greater for metals of high specific resistance.

This fact suggests the inclusion of thermal agitation as a cause of noise. The electrical resistance of the contacts in relative motion is not constant but varies considerably, depending upon the condition of the surfaces, the nature of the contact, the relative velocity and the resistivity of the metals is contact. A simple calculation shows that the thermal agitation voltage in a 250 ohm circuit should be about 0.2 microvolt. This is in the order of the noise originating from the more quiet types of contacts, but is considerably less than is obtained by slightly rough contacts.

are of metal of low resistivity, copper, brass, silver. The best form of contact is that which causes the least relative motion, 1. e. a pressing contact or a rolling contact. In cases where sliding contacts must be used, the surfaces should be so ground and polished that no scraping results, otherwise the surface is roughened and the contacts become noisy. One of the best contact combinations for sliding contact is brass and copper carbon brush material. However, if the surfaces are not ground true, this contact soon develops relatively high noise levels. The noise level with polished nickel silver brushes and contact studs was reduced to -l35 db. by careful working in with 3-in- 1 oil. It is interesting to note that any lubricant will reduce the noise level to some extent, particularly with polished. surfaces of brass or phosphor bronze.

Brass studs and phosphor bronze brushes were used in another modeliIn this case, the noise level was reduced to very low levels but the combination has a tendency to wear and the noise level rises rapidly. A heavy lubricant helps considerably in keeping the noise level low.

With respect to mechanical details, it was found that any construction which allows the bru'shsurface to assume a position at an angle to the plane of the studs is decidely harmful. The construction should be such as to assure uniform pressure between brush and studs at all times. Twisting of the brush results in noise, and any tendency to scrape is decidedly harmful. This means that a wiping" contact is of no use in low level mixing of sound as the nature of the contact must be such that no scraping takes place. Therefore, the action of a wiping contact in scraping the contacts clean is impossible.

The action of silver is particularly noteworthy as the-pieces of silver used were not highly polished. With such contacts a very quiet contact The best contacts from the standpoint of noise could be realized, although the wear might be excessive.

The results obtained in our investigation point to the following items in designing moving contacts for low level mixing units:

MaieriaZs.-The materials of contact should be of low specific resistance and with low thermoelectric potential difference between stud and brush. Copper or materials containing high percentages of copper are exceptionally good, silver is excellent, nickel and chromium or their alloys are poor. A material of interest is copper carbon brush material.

Mechanism.Pressing contact is far superior to sliding contact. In sliding contact, highly polished surfaces with small coefficient of friction are required. Where friction effects are small, they may be helped by the use of a heavy lubricant.

Nise.-The minimum noise level obtained with dry metal contacts (phosphor bronze on brass) was in the order of 160 db. This contact should be lubricated. With self lubricating copper carbon .brushes on brass, the minimum noise level was immeasurably small. With nickel steel studs and brushes, the lowest noise level attained when lubricated with oil was -135 db. Copper carbon brushes on brass, when properly made in a pad having twenty studs were subjected to 2,600 passages over the contact without measurable wear of the brush and contact or increase in noise.

In addition to our discovery of this advantageous contact and brush combination, our invention involves the provision of an improved and simplified assembly of the attenuator resistor elements and control contacts.

The invention will be better understood from the following description when considered in connection with the accompanying drawings and its scope will be pointed out in the appended claims.

Referring to the drawings:

Fig. 1 is a perspective view of the attentuator with its cover removed to show its internal construction,

Fig. 2 illustrates the brush and contact relation of the attenuator, and

Fig. 3 is a wiring diagram illustrating the electrical connections of the attenuator.

As viewed in Fig. 1, the attenuator is seen from the rear without its cover which is, barrel-shaped and is held in place by a screw extending through the casing bottom into the shaft I0 of the attenuator rotor II. The rotor II is operated by means of a knob (not shown) mounted at the front of a panel I2 on which the attenuator is mounted.

The attenuator includes a sub-assembly of three concentrically mounted resistor strips, the outer one I3 of which appears in the drawings. These three concentrically mounted strips are rigidly attached to a contact panel I I which is secured by means of two column supports (not shown) to the panel I2 and which bears two sets of contacts I5 and IG- connected to the input terminals of the attenuator network shown in Fig. 2. Arranged to cooperate with these contacts I5 and I6 are brushes I1 and I8 which are pressed through suitable openings in the rotor II by means of phosphor bronze springs I9 and 20 and are provided with input leads 2| and 22. As previously indicated, the contacts I5 are preferably made of brass and the brushes I1 and ID of carbon copper.

The rotor assembly is attached to the shaft I" by means of a screw 23. Contact is made throughout movement of the rotor by means of contactors and collector rings so that no pigtails are required. Rigidly assembled to the panel l2 are two small terminal boards, only one 24 of which appears in the drawings. Each of these small terminal boards has two soldered terminals. One board is for input and the other is for output connections. There is also a standard ground terminal which is rigidly attached to the panel l2.

The complete device is shielded with a spun aluminum dust cover (not shown). This cover is held in place by means of a knurled thumb screw which engages a tapped hole in the end of the shaft I0. Under the head of this knurled thumb screw is a spring washer in order to insure that the dust cover is definitely held against the rear surface of the nameplate.

The front of the nameplate is etched in such a manner that it indicates 20 steps ranging from zero to 20 in a clockwise direction. The conventional knob with polished nickel pointer is pressed on to the end of the through shaft. The compete assembly is secured to mounting panel by means of four screws located at the corners of the panel l2.

Each of the three concentrically mounted resistor strips is made by securing pieces of terminal wire to a strip of cloth base insulating material such as the phenolic condensation product commercially known as bakelite. After assembly of terminal wires to bakelite strip, the strip is mounted in the conventional resistance windproper number of turns the sections. throughout was No. 40 Ni- Contact is made between the Niare wound on each of ance wire used chrome. chrome where it is desired to obtain an open. The strip is then bent into circular form, and is held together by means of an The circompletely covered with the insulating varnish. The strips are then baked for'one hour at a temperature of 100 degrees F.

The inside strip is then assembled to the bakelite terminal board ire the \following manner. First, all of the terminal 'wires are inserted in cavities present in the ends of brass terminals. This is done by starting at one end of the circular resistor strip and gradually working all of the terminal wires into the proper cavities. Second, soldering of terminal wire to terminal contact is made all the way around on the inside resistance circle.

Next, the center or ladder resistor is put in position concentrically located with relation to the inside resistor strip. Soldered connections are then made to the terminal shanks of the inside resistor ring.

Third, the outside resistance ring is then assembled in place in the same way that the inside resistance ring was assembled.

Fourth, the remaining terminal wires of the middle or ladder resistance ring are then soldered to proper shanks of terminal wires of the outer resistance ring.

The resist- 1 From the foregoing it is evident that the three resistance rings are now in position so that they are all concentric, one with the other, and are in the same plane with relation to the front nameplate. Two pieces of insulation (fish paper) are then inserted rings and these are held in place with three spots of cement.

After the complete assembly of resistance rings to contact panel as already described, the complete contact panel sub-assembly is then "rung on an arbor and a light skin cut is taken across all of the terminals and both collector rings in order to insure face of these parts being exactly at right angles to the center line of the long through shaft bearing. This cut removes little metal from the face of the parts and is taken slowly enough so that it insures good contacting surface in order to do without necessity of a lapping operation. l

The bakelite base for the rotor has two through bushings pressed into it. There is also a main hearing which is pressed into the same bakelite base. After this, both bushings are parallel to the axis of the hole carefully machined so that each and forth freely in its bearing w th minimum amount of clearance. The brushes are held firmly against brass contacts in contact board by means of phosphor bronze springs. In this manner, an even pressure is exerted on the back end of the copper carbon brush which is, in turn, transmitted through to the front conthe brush. Each brush is drilled and in this drilled hole is soldered one. end of a small flexible jumper wire. The other ehd of this wire is soldered to the heel of phosphor bronze contactor which engagesa collector ring, which, in turn, is rigidly secured to contactor panel. Assembly of these parts is such that by rotating phosphor bronze spring, the copper carbon brush can be entirely slipped out of its bearing so that examination of contacting end of brush may be made without unsoldering or dis-:- assembling any parts.

It is to be noted that when this attenuator pad is used in shielded circuits, that shielded input leads come in through slot in one side of the dust cover to input terminal board, while the shieldis grounded by soldered connection on one side of the ground terminal. Shielded output circuit is connected in the same manner with its shield soldered to the remaining side of ground terminal and with the output leads connected to output terminal board. The output leads leave the pad through a second slot diagonally opposite that used by the input leads. In this way a, complete shielded circuit is established throughout.

As shown in Fig. 3, the outer resistor I1 is interconnected with the inner resistor 25 through intermediate resistor sections 26 which are electrically isolated from. one another as previously explained. At one endof the rotor travel the brushes l1 and to are short circuited through a conductor connected to the preceding cross resistor 26 at a point intermediate its ends.

This type of attenuator network has the advantage that (a) It permits construction of a with only two moving contacts.

between these three resistance.

balanced pad (b) The values of the elements are identical "per step and resistance units wound on strips are therefore feasible.

(o) By making the moving contacts the input of the pad whatever noise is generated is delivered as well as the signal.

This attenuator is a finite number of sections of an infinite ladder structure. It can be designed for either mid series or mid shunt termination. With copper carbon brushes in the form of rods, noise levels in the order of -160 db. are obtained. Such levels are in the order of the hiss generated by thermal agitation and it is possible to measure them only approximately. This is done by comparing the threshold of audibility of a known signal with that for the noise of the pad.

The resistance strips are wound of No. 40 .Nichrome" S. C. C. The resistance of each element is held to i 5%. The attenuation per step is 2 db. and the allowable variation from a linear attenuation characteristic is i 0.5 db.

Humidity and life tests made on this pad showed an increase in the noise level for long periods in atmospheres of high humidity. The noise could bereadily reduced by cleaning the contacts. The wear on the brushes is very slight, being immeasurable after 60,000 cycles. However, the noise level increases with use and occasionai cleaning of the brass studs is necessary for extremely quiet operation.

Since at maximum attenuation, almost all the power entering the pad is dissipated in its terminating resistor, the rating oi! this element determines the maximum power that can be controlled by this unit. All resistors are wound of No. 40 Nichrome so that the limiting factor is the operating temperature of the insulating materials. If this is assumed as 40 C. i. e. 100 F. the current entering the pad should not exceed 25 in. a.

We claim:

1. An attenuator including a stator member provided with a pair of sets of brass contacts, a pair of impedance devices concentrically mounted on said member and each connected to a difterent set of said contacts at points spaced from one another, a third impedance device mounted concentrically with said devices and provided with sections connected between said devices at points spaced from one another, input terminals, and a rotor member provided with a pair of copper carbon brushes arranged to be moved along said contacts for variably interconnecting said terminals with said devices.

2. An attenuator including a pair of relative- 1y movable members, a pair, of sets of brass contacts mounted on one of said members, a pair of impedance devices concentrically mounted on said member and each connected to a different set of said contacts, a third impedance device mounted on said member concentrically with said devices and provided with sections connected between said devices at points spaced from one another, and a pair of copper carbon brushes mounted on the other of said members and arranged to successively engage the contacts or each of said pairs:

PHILIP J. HERBST. RALPH H. HEACOCK. PHILIP G. BINDLOSS. 

